1. Field of the Invention
The present invention relates to a multiple-antenna orthogonal frequency division multiplexing (OFDM) system, and more particularly, to a method for increasing diversity gain by applying cyclic delay diversity (CDD) in addition to codeword cycling diversity.
2. Discussion of the Related Art
A multiple-antenna OFDM system requires a variety of techniques to increase the transmission reliability of data. Among them, space-time code (STC), codeword cycling diversity, and CDD aim to increase spatial diversity gain, and beamforming (BF) and precoding are designed to increase signal-to-noise ratio (SNR). STC, code cycling diversity and CDD are used mainly to increase transmission reliability in an open-loop system where feedback information is not available to a transmitter, whereas BF and preceding are used to maximize SNR through feedback information in a closed-loop system where feedback information is available to a transmitter.
CDD uses different delays τ0, . . . , τNt−1 for different antennas, as illustrated in FIG. 1.
Each antenna applies its specific different delay to a time-domain transmission signal, prior to transmission. If signals to be transmitted through the antennas are the same before they are delayed, the multi-path is increased so that the frequency selectivity of a channel is increased. Space frequency block code (SFBC) is one of rank1-transmission schemes among multiple-antenna transmission techniques. SFBC was proposed to use an Alamouti's space time block code (STBC) to the frequency domain.
The codeword cycling scheme applies different preceding vectors or matrices to different time or frequency resources so that a transmission signal experience a plurality of equivalent channels, thereby achieving a diversity gain.
Typically, a diversity scheme needs as many channel estimation pilots as the number of antennas. As a result, pilot overhead increases.